Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
  • H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
  • H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
  • H01L21/3065—Plasma etching; Reactive-ion etching
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
  • B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
  • B81C1/00436—Shaping materials, i.e. techniques for structuring the substrate or the layers on the substrate
  • B81C1/00523—Etching material
  • B81C1/00531—Dry etching
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
  • H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
  • H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
  • H01L21/3065—Plasma etching; Reactive-ion etching
  • H01L21/30655—Plasma etching; Reactive-ion etching comprising alternated and repeated etching and passivation steps, e.g. Bosch process
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
  • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
  • H01L21/3205—Deposition of non-insulating-, e.g. conductive- or resistive-, layers on insulating layers; After-treatment of these layers
  • H01L21/321—After treatment
  • H01L21/3213—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer
  • H01L21/32133—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only
  • H01L21/32135—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only by vapour etching only
  • H01L21/32136—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only by vapour etching only using plasmas
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
  • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
  • H01L21/3205—Deposition of non-insulating-, e.g. conductive- or resistive-, layers on insulating layers; After-treatment of these layers
  • H01L21/321—After treatment
  • H01L21/3213—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer
  • H01L21/32133—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only
  • H01L21/32135—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only by vapour etching only
  • H01L21/32136—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only by vapour etching only using plasmas
  • H01L21/32137—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only by vapour etching only using plasmas of silicon-containing layers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
  • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
  • H01L21/3105—After-treatment
  • H01L21/311—Etching the insulating layers by chemical or physical means
  • H01L21/31105—Etching inorganic layers
  • H01L21/31111—Etching inorganic layers by chemical means
  • H01L21/31116—Etching inorganic layers by chemical means by dry-etching

Definitions

• the present invention relates to the etching of circuit patterns on semiconductor wafer structures in the fabrication of integrated circuits, specifically it relates to the etching of deep trenches in structures in the fabrication of microelectromechanical systems ("MEMS").
• MEMS concerns the technology of very small items, e.g. machines, generally ranging in the size from a micrometer to a millimeter. The potential of very small machines was appreciated long before the technology existed.
• Common applications include inkjet printers operating with piezoelectrics or thermal bubble ejection, accelerometers in cars, e.g. for airbag deployment in collisions, gyroscopes, silicon pressure sensors e.g. for monitoring car tires or blood pressure, optical switching technology or bio-MEMS applications in medical and health-related technologies.
• Microelectronic circuits process the information gathered by sensors from the environment through measuring mechanical, thermal, biological, chemical, optical or magnetic phenomena.
• EP patent application EP-A-O 200 951 discloses a method for high-rate anisotropic etching of silicon by applying a mixture of NF3 or SF5, N2, and a layer forming (passivating) gas such as CHF3.
• British patent GB 2 290 413 discloses a method for processing silicon structures by generating a trench in a first etching step using a gas which supplies fluorine, e.g. SF5 or NF3, and a passivating gas which forms Teflon (RTM), e.g. CHF3, C2F5, C2F4 or C4Fg). Then, underetching is generated by a further isotropic plasma etching step.
• the structures produced may be used as acceleration sensors.
• US patent application publication US 2003/0189024 discloses how to provide openings of variable shape by alternatively etching an opening in silicon and depositing a conformal (passivating) fluorocarbon polymer on the sidewalls.
• HBr and helium-oxygen and optionally SF5 are mentioned as known isotropic etch.
• US patent application publication US 2005/0029221 discloses a process for etching deep trenches in a two-step process. The first step includes etching a tapered trench with a tapered profile. HBr/CF4 is used as preferred etchant in the first step. Then, trimming of the tapered trench by high-density plasma is performed in an isotropic etching step to provide a straight-profile deep trench by applying SF 6 /HBr/O 2 .
• Object of the present invention is to provide an improved process for producing of microelectromechanical systems. Another object of the present invention is to provide novel etching gas mixtures suitable, La., for application in the process of the present invention. These and other objects of the present invention are achieved by the process and gas mixtures as outlined below.
• a method of producing a microelectromechanical system (“MEMS") from a structure comprising a step wherein the structure is etched applying an etching gas comprising elemental fluorine or carbonyl fluoride (COF 2 ) or a mixture of both.
• an etching gas comprising elemental fluorine or carbonyl fluoride (COF 2 ) or a mixture of both.
• the structure is treated with a passivating gas. If etching and passivating are performed in separate steps, preferably a multitude of consecutive etching steps and passivating steps are performed.
• the passivating gas preferably is selected from compounds forming a fluorine-containing or fluoride-containing passivating layer.
• the passivating agent is selected from organic compounds providing, in a plasma, a fluoropolymer; the passivating agent optionally is applied in the presence of hydrogen or a hydrogen-releasing gas.
• the passivating agent optionally is applied in the presence of hydrogen or a hydrogen-releasing gas.
• no hypofluorites, fluoroperoxides and/or fluorotrioxides are introduced into the reactor or intentionally formed therein.
• the gas mixtures are also free of compounds containing a CN bond and hydrogen.
• the items to be etched do not contain WC (tungsten carbide) or its alloys.
• the term “comprising” includes the meaning of “consisting of.
• the “structure” can be, for example, a silicon wafer in bulk micromachining, or an assembled item in surface micromachining.
• the process can be performed thermally, i.e. by performing it at high temperature, e.g. at a temperature equal to or higher than 200 0 C, and equal to or lower than 500 0 C, or according to the cryo process where super low temperatures are involved of up to -110 0 C, the process is best performed using the Bosch process which may be carried out in plasma etchers available from Alcatel, Advanced Materials, Plasma- Therm or Surface Technology Systems.
• the plasma conditions are induced by a magnetron or microwave irradiation.
• Elemental fluorine and carbonyl fluoride can be applied as such, without diluent or additional compounds.
• diluents are selected from the group consisting of nitrogen and noble gases. Further, additives like oxygen, hydrogen, or passivating gases can be added as is described later.
• mixtures of elemental fluorine with nitrogen, helium and/or argon are applied.
• the mixture can be formed in the reactor, or a mixture of elemental fluorine and the inert gas or gases is formed before introducing it into the reactor. If the gases are introduced in such a premixed form into the reactor, a homogenous or near- homogenous mixture can be provided throughout the reactor chamber.
• the fluorine content of the mixture is preferably between 1 and 35 % by volume.
• a mixture of F2 and the inert gas can be supplied from pressurized bottles. In such pressurized bottles, a homogenous mixture forms.
• Mixtures comprising elemental fluorine and nitrogen, mixtures comprising elemental fluorine and argon are preferably applied, and, especially preferably, mixtures comprising elemental fluorine, nitrogen and argon. If mixtures comprising only elemental fluorine and nitrogen are applied, the content of elemental fluorine preferably is equal to or greater than 1 % by volume. The content of elemental fluorine is preferably equal to or lower than 25 % by volume. The content of nitrogen is preferably equal to or lower than 99 % by volume. Preferably, it is equal to or greater than 75 % by volume. In an especially preferred embodiment, the content of elemental fluorine lies in the range of 18 to 22 % by volume.
• the content of argon preferably is equal to or greater than 50 % by volume. Preferably, it is equal to or lower than 99 % by volume.
• the content of elemental fluorine is preferably equal to or greater than 1 % by volume. Preferably, it is equal to or lower than 50 % by volume, especially equal to or lower than 25 % by volume.
• the content of elemental fluorine is preferably equal to or greater than 1 % by volume. It is preferably equal to or lower than 25 % by volume.
• the content of argon is preferably equal to or greater than 4 % by volume. Preferably, the content of argon is equal to or lower than 25 % by volume.
• the content of nitrogen is preferably equal to or greater than 4 % by volume. It is preferably equal to or lower than 75 % by volume.
• the sum of elemental fluorine and argon preferably is equal to or lower than 50 % by volume, especially preferably equal to or lower than 45 % by volume. It is preferably equal to or higher than 25 % by volume.
• mixtures comprising carbonyl fluoride, but being free from elemental fluorine, are applied.
• This embodiment is preferred to the embodiments wherein gas mixtures are applied which contain elemental fluorine (see above) or fluorine and carbonyl fluoride (see below).
• Carbonyl fluoride can be used very flexibly. Generally, it is applied together with other gases or vaporized liquids.
• Carbonyl fluoride also can be used in anisotropic etching and isotropic etching. If it is used in isotropic etching, it can be applied as such, but preferably, it is applied in admixture with other fluorinated organic gases or vapors, for example, saturated or unsaturated fluoroethers or fluorinated esters, for example, those described in JP 10-223614, but especially, it is applied together with nitrogen, oxygen and/or noble gases.
• the carbonyl fluoride content of the mixture is preferably equal to or greater than 1 % by volume. In general, it is equal to or lower than 99 % by volume.
• the mixture can be formed in the reactor, or, which is preferred, a mixture of carbonyl fluoride and the inert gas or gases is formed before introducing it into the reactor. If the gases are introduced in such a premixed form into the reactor, a homogenous mixture is provided throughout the reactor chamber.
• Mixtures comprising carbonyl fluoride and nitrogen, mixtures comprising carbonyl fluoride and argon and mixtures comprising carbonyl fluoride, nitrogen and argon are advantageously applied. They can be applied together with hydrogen or a hydrogen releasing gas. Preferably, these mixtures also include oxygen. If mixtures comprising only carbonyl fluoride and nitrogen are applied, the content of carbonyl fluoride preferably is equal to or greater than 1 % by volume. The content of carbonyl fluoride is preferably equal to or lower than 75 % by volume. The content of nitrogen is preferably equal to or greater than 25 % by volume; preferably, it is equal to or less than 99 % by volume. In one embodiment, the mixtures consist of carbonyl fluoride and nitrogen.
• the content of argon preferably is equal to or greater than 10 % by volume. Preferably, it is equal to or lower than 80 % by volume.
• the content of carbonyl fluoride is preferably equal to or greater than 20 % by volume; preferably, it is equal to or less than 90 % by volume. In one embodiment, the mixtures consist of carbonyl fluoride and argon.
• the content of carbonyl fluoride is preferably equal to or greater than 1 % by volume. It is preferably equal to or lower than 50 % by volume.
• the content of argon is preferably equal to or greater than 5 % by volume.
• the content of argon is equal to or lower than 50 % by volume; especially, the content of argon is equal to or lower than 40 % by volume.
• the content of nitrogen is preferably equal to or greater than 1 % by volume; preferably, it is equal to or less than 80 % by volume.
• the mixtures consist of carbonyl fluoride, nitrogen and argon.
• elemental fluorine and carbonyl fluoride are applied together.
• the molar ratio of elemental fluorine to carbonyl fluoride lies preferably in the range of 1:99 to 99:1.
• the mixture of elemental fluorine and carbonyl fluoride is diluted by oxygen and/or an inert gas, preferably nitrogen, or a noble gas.
• the mixtures preferably are constituted as described above in the first embodiment with the proviso that "mixture of elemental fluorine and carbonyl fluoride" has to be substituted for "elemental fluorine". It is preferred that the sum of the volumes of elemental fluorine and argon are equal to or less than 60 % by volume.
• F2 and COF2 may react with each other, especially to form fluorooxytrifluoromethane, a hypofluorite.
• a reaction preferably such a reaction is not performed.
• no such catalyst is present in the reactor wherein etching is performed.
• the mixtures mentioned above of carbonyl fluoride, elemental fluorine or their mixtures, optionally including nitrogen and/or argon may also comprise oxygen.
• the content of oxygen is equal to or greater than 5 % by volume. Very preferably, it is equal to or greater than 10 % by volume.
• the content of oxygen is equal to or less than 30 % by volume.
• the content of the other constituents is then diminished respectively.
• the content of the other constituents is reduced, for example, by 10 % each.
• elemental fluorine, carbonyl fluoride or the mixture of elemental fluorine and carbonyl fluoride, optionally also comprising nitrogen, noble gases and/or oxygen can be applied together with etching gases known from the art.
• they can be applied together with HBr, SFg or NF3 as additional etching gas.
• the content of the additional gas is preferably less than 20 % by weight of the gas mixture.
• elemental fluorine, carbonyl fluoride or their mixture are applied as the only etching gases, especially preferably together with argon, oxygen and/or nitrogen as described above.
• argon, oxygen and/or nitrogen, respectively are the balance to 100 % by volume.
• the mixtures described above can be applied in MEMS production in anisotropic and isotropic etching.
• anisotropic etching the etching gas forms a trench by etching the bottom of the trench without or nearly without affecting the side walls.
• Anisotropic etching can be induced by balancing physical etching (which essentially etches the bottom of the trench) and chemical etching (which etches bottom and side walls). If isotropic etching is desired, prevention of side wall etching is not necessary. In anisotropic etching, side wall etching is undesired.
• a preferred method of anisotropic etching is performed by either including a passivating gas into the etching gas which passivating gas protects the side walls, or by sequential etching with an etching gas and in a separate step, forming a passivating layer on the side walls using a passivating gas.
• the passivating gas is applied to form a protective layer on the trench walls to prevent them to react with the etchant.
• the nature of the passivating gas depends on the kind of structure.
• bromine sources for example elemental bromine or HBr can be applied as passivating gas.
• a layer of non- volatile Si ⁇ Br ⁇ compounds form as protective layer.
• Other sources forming a protective layer with silicon are NO, NO2, CO2, CO, N2, O2, CS2 or a combination of SiCl4 and N2.
• the passivating gas is preferably selected from cyclic, linear or branched saturated aliphatic compounds with 1 to 6 carbon atoms, substituted by at least one fluorine atom, or from cyclic, linear or branched unsaturated aliphatic compounds with 2 to 6 carbon atoms, substituted by at least one fluorine atom. These compounds consist of carbon and fluorine and optionally hydrogen. Preferably, at least 50 % of the hydrogen atoms of the respective saturated or unsaturated compound are substituted by fluorine atoms. Saturated hydrofluorocarbons and saturated fluorocarbons with 1 to 6 carbon atoms and unsaturated hydrofluorocarbons and unsaturated fluorocarbons with 2 to 6 carbon atoms are preferred.
• Highly preferred compounds which are applicable as passivating gas are C-C 4 F 6 , C-C 5 F 8 , CH 2 F 2 , CHF 3 , CF 4 , C 2 F 6 , C 3 F 8 , C 2 F 4 , C 4 F 6 , and C 4 Fg.
• the compounds c-C ⁇ F ⁇ and CF 3 I are also suitable.
• C 4 F 6 is especially preferred as passivating gas.
• the expert knows that some of the suitable compounds have a boiling point which is higher than 20 0 C at normal pressure. He is aware that for these compounds, the term "gas” should be understood to mean "vapor". These compounds are suitable even if not gaseous at 20 0 C because the process of the present invention is performed at a pressure low enough where these compounds are no longer liquid, but in the form of a vapor. He also is aware that these vapors function like gases.
• fluorocarbon compounds consisting of carbon and fluorine
• hydrogen scavenges fluorine radicals, thus leaving fluorocarbon radicals in the reaction mixture.
• Fluorocarbon radicals are good etchants and still improve the anisotropic effect of the passivating gas; thus, hydrogen seems to make it possible to balance the ratio between fluorine radicals and fluorocarbon radicals.
• a higher share of fluorocarbon radicals compared to fluorine radicals is thought to reduce the reaction with the protective layer on the trench wall.
• Elemental hydrogen or hydrogen providing compounds e.g. hydrogen-containing fluorocarbons, especially Cl or C2 hydrofluorocarbons, most preferably difluoromethane and trifluoromethane, are very suitable.
• the passivating gas can be present even in higher amounts.
• the gas mixture may contain up to 45 % by volume of the passivating compound. The content of one or all of the other constituents is then lower than given above.
• Preferred gas mixtures for passivation only (with low or no anisotropic etching effect) in MEMS preparation include one or more of the above- mentioned aliphatic cyclic, linear or branched fluorocarbons or hydrofluorocarbons with 1 to 6 carbon atoms (saturated) or 2 to 6 carbon atoms (unsaturated).
• a hydrogen or a hydrogen-releasing gas (a gas that releases hydrogen under thermal conditions, especially at temperatures at or higher than 200 0 C, or in a plasma), preferably difluoromethane or trifluoromethane, is also present, especially, if perfluorinated compounds are applied as passivating gas.
• hydrogen or a hydrogen-releasing gas it is preferably comprised in an amount of 1 to 5 % by volume. Also, it is possible to include argon for improving the plasma.
• the process for MEMS etching can principally be performed in two alternatives: the structure is treated with the etching gas and the passivating gas simultaneously, or treatment with the etching gas is performed in one step, and treatment with the passivating gas is performed in another step; the second alternative wherein etching and passivation are consecutively performed is called “Bosch" process.
• the "Bosch" type process is preferred.
• the treatment can be performed as a one-time treatment by extending the treatment time until the desired result, e.g. trench depth, is achieved.
• the steps of etching and passivation are repeated several times consecutively until the desired result is achieved. While it is technically advantageous to perform etching and passivation consecutively, it may be mandatory if etching gas and passivating gas are expected to react with one another in an undesired manner.
• Carbonyl fluoride and C4F5 can be applied in separate steps or together in one simultaneous etching/passivating step, optionally in admixture with nitrogen or a noble gas, e.g. argon, or a hydrogen-releasing gas, because no undesired reaction will occur, elemental fluorine and a perfluorocarbon might also be applied simultaneously, because no adverse reaction is expected (nevertheless, consecutive application may be advantageous because of possible higher speed of reaction), while elemental fluorine and C4F5 should be applied in separate etching and passivating steps, because, even if diluted by nitrogen and/or a noble gas, an addition reaction may take place between elemental fluorine and C4F5.
• a noble gas e.g. argon
• hydrogen-releasing gas e.g. a hydrogen-releasing gas
• etching and passivation are performed in separate steps, then etching can be performed with the etching mixtures describe above. Passivation can then be performed using the passivating gases mentioned above.
• an additional step can be performed to achieve underetching, as described in GB 2 290 413.
• this step it is preferred only to apply the etching gas.
• the temperature of the structure during plasma treatment will generally be kept in a range of 20 0 C to 100 0 C, but it may be higher.
• the pressure during the plasma treatment is preferably equal to 1.5 • 10 ⁇ 2 mbar to 15 mbar.
• the pressure is equal to or greater than 1 • 10" 1 mbar. It is preferably equal to or lower than 1.5 mbar.
• the structure may have variable forms, it is preferably in the shape of a wafer.
• the structure is a silicon wafer.
• the etching can be performed according to the bulk micromachining technology wherein the whole thickness of the item to be etched, e.g. a silicon wafer, is used to build the micromechanical structure.
• the etching can be performed according to the surface micromachining technology wherein layers are produced by applying coatings and selective etching of them.
• the etching process can generally be used in the deep reactive ion etching technology.
• the process according to the present invention can be applied to produce semiconductors for microelectromechanical systems, for example, acceleration sensors, magnetic recording heads, ink jet printers, gyroscopes and other items as described above.
• gas mixtures useful in the process of the present invention are novel; they are also an aspect of the present invention and are claimed per se.
• the term “comprising” includes the meaning of "consisting of.
• These gas mixtures contain no added hypofluorites, fluoroperoxides and/or fluorotrioxide, and preferably, the gas mixtures are essentially free of hypofluorites, fluoroperoxides and/or fluorotrioxide; the term “essentially” denotes a content of any of these compounds of less than 1 % by volume, and preferably a content of 0 % by volume.
• the gas mixtures are also free of compounds containing a CN bond and hydrogen.
• gas mixtures which are described in detail in the following paragraphs are, as mentioned above, claimed per se. They can be in the gaseous state, in the liquid state, in the solid state or even in the supercritical state.
• the gas mixtures claimed per se are preferably present in the liquid state.
• they can be contained, in the liquid state, in pressurized containers, e.g. bottles.
• pressurized containers e.g. bottles.
• a certain amount of gaseous atmosphere exists, depending of the volume of the liquid and the container.
• the mixtures according to the present invention preferably comprise elemental fluorine and carbonyl fluoride in a molar ratio of 1:99 to 99:1.
• the molecular ratio of elemental fluorine and carbonyl fluoride is preferably equal to or greater than 5:95. Preferably, it is equal to or lower than 95:5.
• Mixtures consisting of equimolar amounts of elemental fluorine and carbonyl fluoride are preferably excluded.
• the mixture of elemental fluorine and carbonyl fluoride may further comprise oxygen, a noble gas, nitrogen.
• the noble gas preferably is selected from helium and argon, whereby argon is especially preferred.
• these mixtures are especially suitable for MEMS etching.
• these mixtures may also comprise a passivating gas, preferably a passivating as described above.
• the passivating gas should not react with elemental fluorine; perfluoro compounds are very suitable.
• the amounts of elemental fluorine, carbonyl fluoride, and optionally oxygen, argon, and/or nitrogen and, if comprised, a passivating gas, are given above.
• the mixtures, claimed per se, of elemental fluorine and carbonyl fluoride can be in the gaseous state, in the liquid state, in the solid state or even in the supercritical state.
• the gas mixtures are preferably present in the liquid state. For example, they can be contained, in the liquid state, in pressurized bottles.
• Another embodiment of this aspect of the present invention relates to mixtures comprising carbonyl fluoride, nitrogen and/or a noble gas, optionally a passivating gas and optionally hydrogen or a hydrogen-releasing gas.
• the noble gas is elected from helium and argon, especially preferably it is argon.
• the mixtures consist of carbonyl fluoride and nitrogen, or they consist of carbonyl fluoride and argon, and, in both alternatives, optionally also contain oxygen.
• the content of carbonyl fluoride preferably is equal to or greater than 1 % by volume.
• the content of carbonyl fluoride is preferably equal to or lower than 75 % by volume.
• the content of nitrogen is preferably equal to or greater than 25 % by volume; preferably, it is equal to or less than 99 % by volume.
• the mixtures consist of carbonyl fluoride and nitrogen.
• the content of argon preferably is equal to or greater than 10 % by volume. Preferably, it is equal to or lower than 80 % by volume.
• the content of carbonyl fluoride is preferably equal to or greater than 20 % by volume; preferably, it is equal to or less than 90 % by volume. In one embodiment, the mixtures consist of carbonyl fluoride and argon.
• Mixtures comprising carbonyl fluoride, nitrogen and argon are especially preferred.
• the content of carbonyl fluoride is preferably equal to or greater than 1 % by volume. It is preferably equal to or lower than 50 % by volume.
• the content of argon is preferably equal to or greater than 5 % by volume.
• the content of argon is equal to or lower than 50 % by volume; especially, the content of argon is equal to or lower than 40 % by volume.
• the content of nitrogen is preferably equal to or greater than 1 % by volume; preferably, it is equal to or less than 80 % by volume.
• the mixtures consist of carbonyl fluoride, nitrogen and argon.
• the mixtures may also comprise oxygen, hydrogen, a hydrogen releasing gas and/or a passivating gas. Details are explained above.
• the mixtures, claimed per se, of carbonyl fluoride and nitrogen and/or a noble gas and optionally passivating gas or hydrogen can be in the gaseous state, in the liquid state, in the solid state or even in the supercritical state.
• the gas mixtures are preferably present in the liquid state.
• they can be contained, in the liquid state, in pressurized bottles.
• inventive mixtures are suitable, for example, for performing the process according to the present invention, but also for other etching processes, for example, for producing semiconductors, for flat panel display production or for wafer cleaning.
• Still another embodiment of this aspect of the present invention relates to mixtures comprising carbonyl fluoride and a passivating gas, and optionally also nitrogen, a noble gas and/or a hydrogen-releasing gas. These mixtures are suitable for simultaneous etching and passivating.
• the presence of argon is very preferred.
• the content of carbonyl fluoride preferably is equal to or greater than 15 % by volume, very preferably, equal to or greater than 20 % by volume. Preferably, it is equal to or lower than 60 % by volume.
• the content of the passivating agent is preferably equal to or greater than 10 % by volume, very preferably, equal to or greater than 15 % by volume.
• the content of the passivating gas is preferably equal to or lower than 50 % by volume, very preferably, equal to or lower than 45 % by volume.
• the content of argon preferably is equal to or greater than 20 % by volume, very preferably, equal to or greater than 25 % by volume.
• the content of argon is equal to or lower than 50 % by volume, very preferably, equal to or lower than 40 % by volume.
• nitrogen is comprised, its content is preferably in a range of 1 to 10 % by volume.
• hydrogen or a hydrogen- releasing gas is comprised, its content preferably is equal to or greater than 2 % by volume. Preferably, it is equal to or lower than 15 % by volume.
• the term "passivating gas” denotes inorganic or organic compounds other than N2, CO2, CO, CHF2CI, O2 or CO; suitable passivating gas are compounds which react under thermal (200 0 C and higher) or under plasma conditions with silicon to form silicon compounds with low volatility, or which form a passivating layer thus protecting the structure against etching.
• the term "passivating gas” denotes one or more organic compounds selected from cyclic or linear or branched, saturated aliphatic compounds with 1 to 6 carbon atoms, substituted by at least one fluorine atom, or from cyclic or linear or branched, unsaturated aliphatic compounds with 2 to 6 carbon atoms, substituted by at least one fluorine atom.
• These compounds consist of carbon and fluorine and optionally hydrogen.
• at least 50 % of the hydrogen atoms of the respective saturated or unsaturated compound are substituted by fluorine atoms.
• Saturated hydrofluorocarbons and saturated fluorocarbons with 1 to 5 carbon atoms and unsaturated hydrofluorocarbons and fluorocarbons with 2 to 5 carbon atoms are preferred.
• Highly preferred compounds which are comprised as passivating gas are C-C4F5, c-C5Fg, CH2F2, CHF3, CF4, C2F5, C3Fg, C2F4, C4F5 , and C4Fg.
• C4F5 is especially preferred as passivating gas.
• the compounds c-C ⁇ F ⁇ and CF 3 I are also suitable.
• the mixture of carbonyl fluoride and passivating gas may further include nitrogen, oxygen, and/or a noble gas. Even elemental fluorine can be comprised provided the passivating gas is a perfluorocarbon compound. Still more preferably, the mixture comprises carbonyl fluoride, C4F5, and argon and optionally hydrogen or a hydrogen-releasing gas.
• the mixtures, claimed per se, of carbonyl fluoride and passivating gas can be in the gaseous state, in the liquid state, in the solid state or even in the supercritical state.
• the gas mixtures are preferably present in the liquid state.
• they can be contained, in the liquid state, in pressurized bottles.
• etching gas mixtures including a passivating gas are especially suitable for use in the etching step, the passivating step or the combined etching and passivating step, respectively.
• an etching gas mixture consisting of elemental fluorine, nitrogen and argon and optionally oxygen is a further aspect of the present invention.
• This gas mixture contains equal to or more than 10 % by volume of elemental fluorine. It contains equal to or less, preferably less, than 25 % by volume of elemental fluorine. It contains equal to or more than 5 % by volume of argon. It contains equal to or less than 15 % by volume of argon. It contains equal to or more than 65 % by volume of nitrogen. It contains equal to or less than 80 % by volume of nitrogen. This mixture is very suitable for MEMS etching. If oxygen is contained, it is preferably present in a range between 2 and 15 % by volume.
• the content of one or more of the other constituents may then be lower so that the constituents add up to 100 % by volume.
• the mixtures, claimed per se, of elemental fluorine, nitrogen and argon and optionally oxygen can be in the gaseous state, in the liquid state, in the solid state or even in the supercritical state.
• the gas mixtures are preferably present in the liquid state.
• they can be contained, in the liquid state, in pressurized bottles.
• Example 1 Mixtures suitable for etching silicon in MEMS production
• a silicon wafer for a MEMS device is coated with a photo resist lacquer. After partial exposure of the photo resist lacquer with light according to the desired structure including desired trenches, non-exposed parts of the lacquer are removed. The silicon wafer is then put into a plasma chamber. An premixed etching mixture according to example 1.1 consisting of 20 % by volume of elemental fluorine, 70 % by volume of nitrogen and 10 % by volume of argon is introduced into the chamber at a pressure of about 0.2 mbar, and the microwave radiation is started to initiate plasma conditions. Silicon in regions not covered by the photo resist is etched away isotropically whereby a trench forms in the silicon.
• the etching gas is removed from the reactor, and a passivation gas according to example 1.5 consisting of C 4 Fg (60 % by volume) and argon 40 % by volume) is introduced into the reactor, and the microwave radiation is started to initiate the plasma.
• a passivation gas according to example 1.5 consisting of C 4 Fg (60 % by volume) and argon 40 % by volume
• the hexafluorobutadiene introduced into the reactor essentially forms a fluoropolymer coating on the walls of the trenches formed in the silicon, while the argon stabilizes the plasma.
• the passivating gas is removed, and fresh etching gas is re- introduced into the reactor.
• the silicon layer is then again isotropically etched, thereby deepening the trench formed in the first etching step.
• the passivating layer protects the wall of the trench.
• etching is terminated and the etching gas is removed from the plasma reactor.
• passivating gas is introduced, and another passivating step is performed.
• the passivating gas is removed, and the anisotropic etching is continued. Etching and passivation are consecutively performed until a trench with desired depth has formed.
• the etched wafer can be removed from the chamber.
• Example 5 Preparation of a MEMS device by simultaneous etching and passivating (bulk micromachining)
• a silicon wafer is coated with a dielectric layer of silicon dioxide which, in turn, is coated with a photo resist lacquer. After partial exposure of the photo resist lacquer with light according to the desired structure including desired trenches, non-exposed parts of the lacquer are removed.
• the silicon wafer is then put into a plasma chamber.
• An gas mixture according to example 2.1 consisting of 35 % by volume of elemental carbonyl fluoride, 30 % by volume of argon and 35 % by volume of C4F5 is introduced into the chamber at a pressure of about 0.2 mbar, and the microwave radiation is started to initiate plasma conditions. Silicon dioxide in regions not covered by the photo resist is etched away. During etching, a trench forms.
• a fluoropolymer passivation layer is formed on the walls of the trench.
• the treatment is continued until the trench has the desired depth.
• the etching/passivating gas is removed from the reactor, and the etched silicon wafer can be removed from the chamber.
• Example 6 Preparation of a MEMS device by simultaneous etching and passivating with a gas mixture comprising a hydrogen-releasing gas (bulk micromachining)
• Example 5 is repeated with the gas mixture of example 2.2 consisting of 40 % by volume of carbonyl fluoride, 20 % by volume of C4F5, 30 % by volume of argon and 10 % by volume of CHF3.
• Example 7 Preparation of a MEMS device (surface micromachining)
• a round silicon wafer disk forms the basis of the MEMS to be produced.
• a silicon nitride layer is formed on the wafer by means of a low-pressure CVD (Chemical Vapor Deposition) process.
• a sacrificial layer of silicon dioxide is deposited, again by means of LPCVD.
• the sacrificial layer allows a beam to be freely moving on the MEMS device.
• the sacrificial layer is opened, e.g. by wet etching with HF/NH4F, to allow anchoring of the planned beam.
• a polysilicon layer is deposited by means of LPCVD.
• a lithographical mask is applied, and then, an etching gas of table 1, e.g. the gas mixture of example 1.1, is applied in a plasma chamber to remove the undesired parts of the polysilicon layer on the wafer. Then, another layer is applied, e.g. photoresist SU8, an epoxy-based resin used as structural material. Details about SU8 and how to apply it can be found
• Gold as means for heating the MEMS is applied in a lift-off process and is deposited by electronic-beam evaporation using lithography to remove undesired parts of the SU8 layer together with deposited gold thereon, so that only the desired gold structure remains.
• the sacrificial layer is now removed, e.g. with the wet etching mixture mentioned above. Now, the beam is released, and the structure is finished.

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